Scoreattack is your best bet. A couple of Vetos to get the right map then just take what you can get.

It was the Winter... - Complete Winter Contingency on Legendary without dying - 3000cR

Even without the Save & Quit before the end, this is an easy challenge. Stick to the right, stay in the Jeep, let the other guys handle the fighting. In that order, just make sure to have a Plasma Pistol & DMR on you.

Up Close and Personal - Kill 5 enemies with close quarters combat in a multiplayer Matchmaking game - 2343cR